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-------- We examine an approach for providing reliable, scalable multicast communication, involving the use of multiple multicast groups for reducing receiver processing costs in a multicast session. In this approach a single multicast group is used for the original transmission of packets. Retransmissions of packets are done to separate multicast groups, which receivers dynamically join or leave. We first show that protocols using an infinite number of multicast groups incur much less processing overhead at the receivers compared to protocols that use only a single multicast group.This is due to the fact that receivers do not receive retransmissions of packets they have already received correctly. Next, we derive the number of unwanted redundant packets at a receiver due to using only a finite number of multicast groups, for a specific negative acknowledgment (NAK)--based protocol. We then explore the minimum number of multicast groups required to keep the cost of processing unwanted packets to a sufficiently low value (i.e., to achieve most of the benefit of using an infinite number of multicast groups). For an application consisting of a single sender transmitting reliably to many receivers we find that only a small number of multicast groups are required for a wide range of system parameters. In the case of an application where all participants simultaneously act as both senders and receivers a moderate number of multicast groups is needed. Finally, we present a local filtering scheme for minimizing join/leave signaling when multiple multicast groups are used.